Method and apparatus for stability control of open loop motor drive operation

ABSTRACT

Motor drives and control methods are presented for sensorless motor speed control in which inverter output currents are sampled from the inverter output, and a frequency modulation value is determined based on the current feedback and either one or more voltage commands or one or more voltage feedback signals. A speed or frequency setpoint is adjusted at least partially according to the frequency modulation value to provide an adjusted frequency or speed setpoint value that is then used in controlling the inverter to provide stability control to mitigate hunting or motor stoppage.

REFERENCE TO RELATED APPLICATION

Reference is made to U.S. patent application Ser. No. 13/868,216, filed Apr. 23, 2013 and entitled “Position Sensorless Open Loop Control for Motor Drives with Output Filter and Transformer”, the entirety of which is hereby Incorporated by reference.

BACKGROUND

Sensorless motor drives are used in a variety of applications, particularly where providing position and/or speed sensors directly at a motor load is difficult or impractical. A typical sensorless system employs a voltage-frequency (V/F, alternatively known as Volts per Hertz, V/Hz) controller providing a voltage setpoint according to a desired motor speed or frequency, and this form of sensorless control has been used primarily with induction motors. In certain applications, a step-up transformer may be used to boost the motor drive output voltage, allowing use of a low-voltage drive to power a medium voltage induction motor, and/or to reduce I²R losses and facilitate use of a smaller diameter cable wire for long cable runs between the motor drive and the driven motor. Certain applications also employ sine wave filters, such as LC filters to suppress reflected wave voltage spikes associated with pulse width modulated (PWM) variable frequency drives. Use of sensorless voltage-frequency control techniques, however, may lead to problems, particularly where a transformer and/or sine wave filter is connected between the motor drive and the motor load. Conventional sensorless field-oriented-control (FOC) or other open loop speed control techniques have thus been found generally unsuitable for low-speed motor drive operation where output filters and transformers are used, such as in electric submersible pumps (ESPs), and these difficulties are particularly problematic in driving permanent magnet synchronous motors (PMSMs). Moreover, voltage-frequency control in combination with a sine wave filter under starting conditions may result in the motor not being able to start, with large oscillations on the rotor shaft for low frequency commands. Motors in sensorless speed control applications also suffer from oscillation in rotor velocity about the setpoint speed following load transitions or speed setpoint adjustments, particularly at low speeds. In certain situations, moreover, the driven motor may be unable to successfully start from a stopped condition due to unstable motor speed oscillations. Thus, improved techniques and motor drives are needed for sensorless motor speed control, particularly for driving permanent magnet motors to provide improved stability control.

SUMMARY

Various aspects of the present disclosure are now summarized to facilitate a basic understanding of the disclosure, wherein this summary is not an extensive overview of the disclosure, and is intended neither to identify certain elements of the disclosure, nor to delineate the scope thereof. Rather, the primary purpose of this summary is to present various concepts of the disclosure in a simplified form prior to the more detailed description that is presented hereinafter. The present disclosure provides motor drive apparatus and control techniques by which improved motor speed stability can be facilitated, and finds particular utility in association with open loop or sensorless speed control applications involving asynchronous or synchronous motors, directly driven or in combination with output filters and/or output transformers. The disclosed techniques, moreover, can be employed in systems using voltage-frequency and/or current-frequency control algorithms.

Methods are disclosed in accordance with one or more aspects of the present disclosure for sensorless motor speed control in a motor drive. The methods include sampling one or more AC output current feedback signals or values from the output of the motor drive inverter, and computing a frequency modulation value based on the output current feedback and on one or more voltage command or voltage feedback signals or values. In addition, the methods involve adjusting a frequency or speed setpoint value based in whole or in part on the frequency modulation value in order to provide an adjusted frequency or speed setpoint value, as well as controlling the motor drive inverter according to the adjusted frequency or speed setpoint value. The original speed or frequency setpoint may, in certain embodiments, be rate limited prior to adjustment according to the modulation value. In addition, the adjustment may involve subtracting the modulation value from the rate limited or original frequency or speed setpoint value. In certain embodiments, moreover, the modulation value computation may include computing an estimated motor performance value, such as estimated electromagnetic torque, torque angle, power factor, power factor angle, power, or error between the open loop angle and an estimated rotor position, or other performance value related to operation of the motor which can be estimated according to the current feedback and one or more voltage signals. In addition, the estimated motor performance value may be high pass filtered in certain embodiments to remove any DC offset prior to adjustment of the received speed or frequency setpoint.

Motor drives are provided in accordance with further aspects of the disclosure, including an inverter which provides AC output power to drive a motor load, as well as a sensorless motor speed controller that provides switching control signals to operate the inverter based at least in part on a frequency or speed setpoint. The motor speed controller implements a signal generator which computes a frequency modulation value according to one or more AC output current feedback signals or values representing the output current of the inverter, as well as according to one or more voltage command signals or values used in controlling the inverter. In addition, the motor speed controller includes an adjustment component which adjusts the frequency or speed setpoint value based in whole or in part on the frequency modulation value in order to provide an adjusted frequency or speed setpoint value, as well as a command generator which provides switching control signals to the inverter at least partially according to the adjusted frequency or speed setpoint value.

In certain embodiments, the signal generator computes an estimated motor performance value based on the AC output current feedback and on one or more voltage command or voltage feedback signals or values, and computes the frequency modulation value according to the estimated motor performance value. In addition, the motor speed controller may include a high pass filter for filtering the estimated motor performance value, and the resulting filtered estimated motor performance value is used in computing the frequency modulation value in certain embodiments. The signal generator in certain embodiments computes the estimated motor performance value as an estimated torque value, power factor value, power factor angle value or power value. In certain embodiments, the sensorless motor speed controller includes a rate limiter which operates to rate limit the frequency or speed setpoint, and the adjustment component subtracts the frequency modulation value from the rate limited frequency or speed setpoint value to provide the adjusted frequency or speed setpoint value for operating the inverter. The adjustment component in certain embodiments subtracts the frequency modulation value from the frequency or speed setpoint value in order to provide the adjusted frequency or speed setpoint.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and drawings set forth certain illustrative implementations of the disclosure in detail, which are indicative of several exemplary ways in which the various principles of the disclosure may be carried out. The illustrated examples, however, are not exhaustive of the many possible embodiments of the disclosure. Other objects, advantages and novel features of the disclosure will be set forth in the following detailed description when considered in conjunction with the drawings, in which:

FIG. 1A is a schematic diagram illustrating an exemplary motor drive with a stability signal generator creating a frequency modulation value for adjusting a speed or frequency setpoint to create inverter switching control signals for enhanced stability in sensorless motor speed control according to one or more aspects of the present disclosure;

FIG. 1B is a schematic diagram illustrating a motor drive with a stability signal generator used in sensorless speed control of a motor connected to the inverter output through a sine wave filter and a transformer;

FIG. 2 is a schematic diagram illustrating an exemplary motor drive controller with a stability signal generator and a voltage-frequency control configuration;

FIG. 3 is a schematic diagram illustrating another exemplary motor drive controller with the stability signal generator and a current-frequency controller;

FIG. 4 is a schematic diagram illustrating yet another motor drive controller with a stability signal generator and a current-frequency controller with a reduced bandwidth proportional-integral (PI) controller for driving a motor through a sine wave filter and transformer;

FIG. 5 is a schematic diagram illustrating an exemplary frequency modulation (FM) signal generator in the motor drive controller used for computing an estimated electromagnetic torque value based on estimated stator flux;

FIG. 6 is a schematic diagram illustrating another exemplary frequency modulation signal generator used to compute an estimated electromagnetic torque value based on estimated rotor flux;

FIG. 7 is a schematic diagram illustrating yet another FM signal generator which computes an estimated power factor value for selective adjustment of the speed or frequency setpoint;

FIG. 8 is a schematic diagram illustrating another exemplary FM signal generator computing an estimated power factor angle for use in adjusting the speed or frequency setpoint;

FIG. 9 is a schematic diagram illustrating still another FM signal generator computing an estimated power value for adjusting the speed or frequency setpoint;

FIG. 10 is a graph illustrating a stationary reference frame transformation in the motor drive controller;

FIG. 11 is a graph showing motor speed, current and load torque curves for the motor drive without stability control for a step load change;

FIG. 12 is a graph showing motor speed, current and load torque curves for the motor drive using the stability control concepts of the present disclosure;

FIG. 13 is a flow diagram illustrating an exemplary method for sensorless motor speed control in the motor drive in accordance with further aspects of the present disclosure.

DETAILED DESCRIPTION

Referring now to the figures, several embodiments or implementations are hereinafter described in conjunction with the drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale.

Methods 100 and motor drive apparatus 10 are presented below for sensorless or open loop motor speed control using a stability signal generator as part of the motor drive controller. The control techniques and motor drives may be used in a variety of applications, including without limitation driving electric submersible pumps, whether including an induction motor or a permanent magnet synchronous motor (PMSM), and may be employed in situations in which the motor drive is directly connected to the driven motor or where one or more intervening components (e.g., sine wave filters and/or transformers) are connected between the output inverter and the driven motor. In addition, the concepts of the present disclosure may be employed in conjunction with any suitable form of control command algorithm, including without limitation voltage-frequency and/or current-frequency control, using any suitable internal closed loop feedback regulation configuration (e.g., proportional-integral or PI control components). The presently disclosed techniques, moreover, may be successfully employed to improve stability in operating permanent magnet and/or induction motors to avoid or mitigate undesirable “hunting” (oscillation) and/or undesired motor stopping or inability to start, particularly for low speed operation and/or in the presence of changes or perturbations in the load and/or desired setpoint operating speed. In this regard, while illustrated and described in various exemplary embodiments below, the various aspects of the present disclosure, and in particular the stability control techniques may be used in a wide variety of motor drive applications, motor drive controller architectures, etc., and the various concepts are not limited by the illustrated embodiments.

Referring initially to FIGS. 1A and 1B, various exemplary motor drives 10 are presented hereinafter, in sensorless or open loop speed control configurations wherein no direct measurement of the motor speed or position is provided. While these circumstances are common in remotely driven motor situations (e.g., submersible pump applications, etc.), the various stability control aspects of the present disclosure may also be employed in systems in which the actual motor speed is directly measured and feedback signals are provided to the motor drive controller. As shown in FIGS. 1A and 1B, a power conversion system 2 generally includes an AC power source 4 providing single or multiphase power (e.g., 480 V AC, 50 or 60 Hz) to a motor drive power converter 10. The motor drive 10, in turn, includes one or more power conversion stages with an inverter 14 providing single phase or multiphase AC output currents (e.g., three-phase output currents IA, IB and IC in the illustrated examples) to drive a motor load 6. As shown in FIG. 1A, the motor load 6 may be directly driven by the output of the motor drive inverter 14, or one or more intervening circuits may be connected between the inverter 14 and the motor load 6, such as one or both of a sine wave filter 16 and/or a transformer 18 and a potentially lengthy cable 8 as illustrated in FIG. 1B.

The drive 10 in these examples includes an active or passive rectifier 12 providing rectification of the received (e.g., three-phase) AC input power to create a DC bus voltage across a DC link circuit 13 including a capacitance C. Any suitable form of rectifier 12 can be used, including without limitation a passive rectifier (e.g., one or more rectifier diodes), or a switching rectifier operating at or near the fundamental frequency of the AC input source (fundamental front end or FFE) or at a higher and possibly variable switching frequency, such as an active front end (AFE) rectifier that performs additional functions such as power factor correction, etc. The DC link circuit 13 provides a DC input voltage to a switching inverter 14. The inverter 14 in this example includes switching devices S1, S2, S3, S4, S5 and S6 operated according to inverter switching control signals 22 from a controller 20 to convert the DC power to AC output currents IA, IB and IC for driving the motor load 6. Although the illustrated inverter 14 provides a three-phase two-level output, other single or multiphase or multilevel output implementations are possible within the scope of the present disclosure. Any suitable inverter switching devices S1-S6 may be used, including without limitation insulated gate bipolar transistors (IGBTs), silicon controlled rectifiers (SCRs), gate turn-off thyristors (GTOs), integrated gate commutated thyristors (IGCTs), etc. The controller 20 and the elements and components thereof can include suitable logic or processor-based circuitry, and may also include signal level amplification and/or driver circuitry (not shown) to provide suitable drive voltage and/or current levels sufficient to selectively actuate the switching devices S1-S6, for instance, such as comparators, carrier wave generators or digital logic/processor elements and signal drivers, etc. Moreover, the controller 20 can provide the switching control signals 22 according to any suitable pulse width modulation technique, including without limitation space vector modulation (SVM), carrier-based pulse width modulation, selective harmonic elimination (SHE), etc. In addition, the controller 20 implements various computational functions as detailed hereinafter and may include suitable analog to digital and digital to analog conversion components and processor-based or otherwise programmable logic circuitry alone or in combination with analog circuitry to perform various operations on signals or values as described herein. Furthermore, certain embodiments of the controller 20 may include processing elements and electronic memory for storage of data and program instructions by which the controller 20 may implement the various methods (e.g., method 100 below) detailed herein.

The system 2 in FIG. 1B further includes a sine wave or output filter 16, in one example, a three-phase LC filter having a series filter inductor LF in each output line, as well as a corresponding filter capacitor CF coupled between the corresponding phase line and a common connection point. Other output filter topologies may be used, such as LCL filters, CLC filters, etc. with one or more series elements and further filter elements (e.g., filter capacitors CF) connected in any suitable delta or Y configuration. The example of FIG. 1B also includes a transformer 18 between the filter 16 and the motor cable 8. The illustrated transformer 18 has a three phase delta-connected primary as well as a

Y-connected secondary, although any suitable transformer primary and/or secondary winding configuration or topology may be used. Moreover, the transformer 18 may in certain embodiments be a step-up transformer. For example, a step-up transformer 18 may facilitate use of a low-voltage drive 10 to power a medium or high voltage motor 6, or allow use of a medium-voltage drive 10 to power a high-voltage motor 6. Also or in combination, a step-up transformer 18 may be useful to allow a reduction in the current levels carried by the cable 8, thereby facilitating use of smaller diameter cable wires in the cable 8. The cable 8, moreover, can be of any suitable construction for interfacing the motor drive output, the sine wave filter 16, and/or the transformer 18 with the leads of the motor 6.

The motor drive 10 and the controller 20 operate in sensorless fashion to control one or more operating parameters of the driven motor load 6. For example, the controller 20 provides the inverter switching control signals 22 in order to control position and/or speed and/or torque of the motor 6 without directly sensing any of these controlled parameters. In the illustrated implementation, for instance, current sensors 27 are disposed at the output of the inverter 14 to provide feedback signals or values 28 (e.g., i_(a), i_(b) and i_(c)) to the controller 20 which represent the inverter output currents IA, IB and IC, and/or from which the values of these output currents can be computed, derived or otherwise estimated. Any suitable current sensing devices 27 can be used to generate or provide the signals and/or values 28, and may provide analog signals 28 and/or the sensors 27 may be smart sensors providing digital values 28 representing the output currents IA, IB and IC generated at the output of the inverter 14.

The controller 20 uses the feedback signals or values 28 as well as one or more desired operating parameters to regulate or adjust the output currents IA, IB and IC. Overall, however, the control technique implemented by the illustrated controller 20 is essentially sensorless or open-loop with respect to the actual operating condition of the driven motor load 6 (e.g., speed and/or position), as there are no direct feedback signals obtained from the motor 6 itself. In the examples of FIGS. 1A and 1B, for instance, the controller 20 receives the desired frequency or motor speed value f*21 from a supervisory control system component (not shown), which may be a distributed control system (DCS) element, a user-adjustable knob, local user interface, etc. The controller 20, moreover, includes a voltage command generator component 23, which can be a voltage-frequency 24 or current-frequency control component 24 a (e.g., FIGS. 2-4 below). There may also be a closed loop controller such as a proportional-integral (PI) controller 26 which may be a reduced bandwidth PI controller in certain embodiments (e.g., FIG. 4 below). In operation, the control components 24 a and 26 in FIG. 3 are used to regulate the inverter output currents IA, IB and IC by providing the inverter switching control signals 22 according to the desired speed or frequency signal or value 21 and the feedback signals or values 28.

In addition, in accordance with one or more aspects of the present disclosure, the controller 20 implements a stability signal generator component 25 which computes a frequency modulation value 25 a (e.g., Δf*) based on the current feedback signal or value i_(a), i_(b) and i_(c) as well as on one or more voltage command signals or values 37 v_(a)*, v_(b)* and v_(c)* used in controlling the inverter 14. As seen in FIGS. 1A and 1B, for instance, the voltage command generator component 23 internally generates the voltage commands v_(a)*, v_(b)* and v_(c)* and provides these to the stability signal generator component 25 for generating the frequency modulation value 25 a. The sensorless motor speed controller 20 also includes an adjustment component 29 (e.g., a summer in one example) which adjusts the frequency or speed setpoint value 21 based at least partially on the frequency modulation value 25 a to provide an adjusted frequency or speed setpoint value 31 a (e.g., f_(AD) in FIGS. 2-4 below), and the command generator 23 provides the switching control signals 22 to the inverter 14 at least partially according to the adjusted frequency or speed setpoint value 31 a. In the illustrated example, the adjustment component 29 subtracts the frequency modulation value 25 a from the frequency or speed setpoint value 21 to provide the adjusted frequency or speed setpoint value 31 a. Other implementations are possible in which an adjustment component 29 performs a different type of adjustment, such as offsetting, multiplication, division, addition, or combinations thereof, etc., wherein the concepts of the present disclosure are not limited to the illustrated subtraction example.

Referring also to FIGS. 2-4, FIG. 2 shows further details of an exemplary motor drive controller 20 with a stability signal generator 25 and a voltage-frequency control configuration implemented in the voltage command generator 23. The voltage command generator 23 includes a voltage-frequency (V-F) control component 24 which receives the adjusted frequency setpoint 31 a from the adjustment component 29 and generates a δ-axis voltage setpoint (v*_(δ)) 35 accordingly. The controller 20 implements various components, for example, in processor-executed software or firmware, and operates on certain variables in a synchronous δ, γ reference frame, with received feedback signals or values 28 and generated switching control signals 22 being reference to a stationary (e.g., a, b, c) reference frame. In this regard, the illustrated δ, γ reference frame rotates at the same frequency as the conventional field commutation control (D, Q) reference frame, but the position need not be the same, with γ and δ somewhat analogous to “d” and “q”, but they are not necessarily aligned (e.g., γ will likely be somewhere between the D axis and the Q axis, and γ and δ are orthogonal to one another). It is also understood that the described voltage and/or current regulation can be performed in other reference frames. The voltage-frequency control component 24 in the example of FIG. 2 provides a voltage setpoint output 35 based on the adjusted frequency or speed setpoint signal or value 31 a. As seen in FIG. 2, moreover, a rate limiter 30 may optionally be included to rate limit the frequency or speed setpoint value 21 to provide a rate limited frequency or speed setpoint value 31 (f_(RL)), and the adjustment component 29 in this example subtracts the frequency modulation value 25 a from the rate limited frequency or speed setpoint value 31 to provide the adjusted frequency or speed setpoint value 31 a as an input to the control component 24.

The illustrated voltage-frequency control component 24 implements a multiple-range curve or function as illustrated, with the voltage-frequency relationship having a zero voltage value V_(BOOST) corresponding to a zero frequency value (e.g., 0 Hz), and remains constant at V_(BOOST) until a predetermined boost frequency F_(BOOST). The voltage-frequency relationship implemented by the control component 24 also includes a second portion with increasing voltage values corresponding to a frequency range from the boost frequency F_(BOOST) to a cutoff frequency value F_(CUT), as well as a higher portion with a constant voltage value (e.g., V_(MAX)) corresponding to frequencies above the cutoff frequency F_(CUT), where V_(MAX) can be the maximum rated output voltage of the inverter 14 in certain implementations, and the cutoff frequency F_(CUT) is preferably set to correspond to the rated operating frequency of the motor 6 (e.g., about 60 Hz in one implementation). The control component 24 in certain embodiments can be implemented using a lookup table or a parametric function. The voltage-frequency output v*_(δ) is the δ axis voltage setpoint 35. In the illustrated embodiment, the controller 23 operates according to a zero γ axis value 34 (v*_(γ)=0), although not a strict requirement of all implementations of the present disclosure.

As seen in FIGS. 2 and 10, moreover, the illustrated stability signal generator 25 includes stationary reference frame converter components 50 and 51 for respective conversion of the current feedback signals 28 and the voltage command signals or values 37 from the a,b,c reference frame to a stationary “α,β” reference frame by the illustrative transformation shown in FIG. 10. In this manner, the converter 50 provides α,β reference frame current feedback signals i_(α) and i_(β), and the converter 51 provides voltage command signals v_(α)* and v_(β)* to a frequency modulation (FM) signal generator component 52. In the embodiment of FIG. 2, moreover, the signal generator component 25 employs the FM signal generator component 52 to compute an estimated motor performance value 52 a, in this case an estimated (per unit) torque value {umlaut over (T)}_(pu), based on the AC output current feedback i_(a), i_(b), i_(c) (converted to signals or values i_(α) and i_(β)) and also based on the voltage command signals or values v_(a)*, v_(b)*, v_(c)* (e.g., converted signals or values v_(α)* and v_(β)*). In particular, the signal generator 52 includes a first component 53 which calculates estimated rotor flux linkage values {circumflex over (λ)}_(αr), {circumflex over (λ)}_(βr) based on the output current feedback i_(α) and i_(β), the voltage commands v_(α)* and v_(β)*, and on an output load resistance value R corresponding to the resistance of the motor 6 and any intervening cable 8 according to the following formulas (1) and (2):

{circumflex over (λ)}_(αr) =−i _(α) L+∫(v* _(α) −i _(α) R), and   (1)

{circumflex over (λ)}_(βr) =−i _(β) L+∫(v* _(β) −i _(β) R),   (2)

where L is the inductance of the motor and any intervening cable.

In addition, the signal generator 52 includes a second component 54 which is operative to compute the estimated torque value {umlaut over (T)}_(pu) based on the estimated rotor flux values {circumflex over (λ)}_(αr) and {circumflex over (λ)}_(βr), as well as on the current feedback i_(α) and i_(β) according to the following equation (3):

$\begin{matrix} {{{\hat{T}}_{pu} = \frac{3\; {P\left( {{{\hat{\lambda}}_{\alpha \; r}i_{\beta}} - {{\hat{\lambda}}_{\beta \; r}i_{\alpha}}} \right)}}{2\; T_{rated}}},} & (3) \end{matrix}$

where P is the number of pole pairs in the motor 6 and T_(rated) is the rated torque for the motor 6. In certain embodiments, the estimated torque value {umlaut over (T)}_(pu) can be computed using stator flux values, and may be computed according to rotor flux values in other embodiments.

The estimated torque or other estimated motor performance value 52 a is then used by the signal generator component 25 to compute the frequency modulation value 25 a. As seen in FIG. 2, moreover, the illustrated embodiment includes a high pass filter 55 to filter the estimated motor performance value 52 a to provide a filtered estimated motor performance value 55 a, thereby removing undesirable DC offsets from the estimated motor parameter 52 a. The signal generator 25 then computes the FM value 25 a based at least partially on the filtered estimated motor performance value 55 a. In the illustrated example, a gain component 56 is provided to amplify the filtered signal 55 a, and a limiter circuit 57 may be included to limit the range of the amplified signal. The illustrated embodiment also includes a multiplier 59 which multiplies the output of the limiter 57 by a sign signal from a sign function 58 based on the sign (positive or negative, i.e. forward or reverse) of the desired frequency or speed setpoint input signal or value 21. The output of the multiplier 59 provides the frequency modulation value 25 a to the summing junction adjuster component 29 for adjustment of the received (and optionally rate limited) speed or frequency setpoint to create the adjusted setpoint 31 a.

The adjusted setpoint 31 a provides an input to the voltage command generator 23 and the control component 24 thereof. The V-F controller 24, moreover, provides γ and δ axis voltage setpoint signal or value outputs v_(γ)* 34 and v_(δ)* 35, respectively, which are converted to the stationary reference frame by a converter 36 (δ,γ to a,b,c) using a phase angle signal or value θ 43 obtained from the adjusted frequency setpoint 31 a via a multiplier 40 (2π) providing a frequency output 41 (ω) and an integrator 42 to create the angle output 43. The reference frame converter 36, in turn, provides the set of three stationary reference frame voltage setpoint signals or values 37 (v_(a)*, v_(b)* and v_(c)*) as inputs to a pulse width modulation (PWM) component 38 that includes any suitable form of modulation, isolation, amplifiers, driver circuitry, etc. to generate the inverter switching control signals 22 for controlling the inverter 14 using known techniques.

FIG. 3 illustrates another controller embodiment 20 in which the stability signal generator 25 again provides the frequency modulation signal 25 a for stability control based on an estimated torque value 52 a. In this case, however, the voltage command generator 23 includes a current-frequency controller 24 a along with a PI controller 26. In this case, moreover, an a,b,c to δ,γ converter 44 provides δ,γ current feedback signals i_(γ) 48 and i_(δ) 46 as inputs to the PI controller 26 for closed loop regulation with respect to its current setpoints i_(δ)* 32 and i_(γ)* 33 (=0). The current-frequency (I-F) control component 24 a in this example receives the adjusted frequency or speed setpoint 31 a, and generates the δ-axis current setpoint (i*_(δ)) 32 accordingly. In one possible implementation, the current-frequency control component 24 a implements a dual-range curve or function as illustrated, with the current-frequency relationship being a zero current value corresponding to a zero frequency value (e.g., 0 Hz). As shown in FIG. 3, the current-frequency relationship implemented by the control component 24 a includes a first portion with increasing current values corresponding to a first frequency range from the zero frequency value to a cutoff frequency value F_(CUT), as well as a second portion with a constant current value (e.g., I_(MAX)) corresponding to frequencies above the cutoff frequency F_(CUT), where I_(MAX) can be the maximum rated output current of the inverter 14 in certain implementations, and the cutoff frequency F_(CUT) is preferably set to correspond to a very low operating frequency of the motor 6 (e.g., about 0.5-1.0 Hz). The current-frequency control component 24 a in certain embodiments can be implemented using a lookup table or a parametric function. In this regard, the current-frequency relationship includes the first ramped portion until the cutoff frequency, after which the maximum current is demanded, whereby the control component 24 a avoids sending DC current to any included output transformer 18 (e.g., FIG. 4 below) at zero frequency.

The output (i*_(δ)) of the current-frequency controller 24 a is the δ axis current setpoint 32, which is provided to the PI control component 26. PI control is not a strict requirement of all embodiments of the present disclosure, wherein any suitable current regulating algorithm can be used to regulate the inverter output currents IA, IB and IC, and the PI algorithm in certain embodiments may have an algorithm bandwidth that is less than the resonant frequency of any included sine wave filter 16 (e.g., FIG. 4 below). In the illustrated embodiment, the PI controller 26 operates according to a zero γ axis value 33 (i*_(γ)=0), although not a strict requirement of all implementations of the present disclosure.

In the example of FIG. 3, moreover, the FM signal generator 52 includes components 53 and 54, in this case computing an estimated motor performance value 52 a as an estimated torque based on estimated stator flux linkage values via the component 53 according to a motor resistance value R_(motor) associated with the driven motor 6 according to the following equations (4) and (5):

{circumflex over (λ)}_(αs)=∫(v _(α) −i _(α) R _(motor)), and   (4)

{circumflex over (λ)}_(βs)=∫(v _(β) −i _(β) R _(motor)).   (5)

where v_(α) and v_(β) are measured motor voltages after the a,b,c to α,β transformation 51. In addition, the component 54 computes the estimated torque value according to the following equation (6):

$\begin{matrix} {{\hat{T}}_{pu} = {\frac{3\; {P\left( {{{\hat{\lambda}}_{\alpha \; s}i_{\beta}} - {{\hat{\lambda}}_{\beta \; s}i_{\alpha}}} \right)}}{2\; T_{rated}}.}} & (6) \end{matrix}$

FIG. 4 shows another motor drive controller 20 with a stability signal generator 25 and a voltage command generator 23 generating voltage commands, where the included PI controller is a reduced bandwidth proportional-integral controller for driving a motor 6 through a sine wave filter 16 and transformer 18. As in the examples above, moreover, the signal generator 25 computes an estimated per unit torque value {umlaut over (T)}_(pu) for deriving the frequency modulation value 25 a. The inventors have appreciated that limiting the bandwidth of the PI controller 26 avoids or mitigates large inrush current during power up, particularly where the drive 10 is providing output currents through a sine wave filter 16 and/or transformer 18. In certain applications, for instance, the sine wave filter 16 makes the inverter output particularly susceptible to large inrush currents, and limiting the bandwidth of the PI controller 26 (or other current regulation control algorithm implemented by the controller 20) to be well below the sine wave filter resonant frequency helps to mitigate or avoid high inrush current levels, particularly at power up.

Referring also to FIGS. 5-9, the above described examples use a frequency modulation signal generator 52 that computes estimated torque values 52 a which are optionally high pass filtered using the filter 55 and scaled and limited appropriately via the components 56, 57, and sign-adjusted via components 58 and 59 to provide the frequency modulation value 25 a. Various other embodiments are possible, some of which are illustrated in FIGS. 5-9, in which the value 25 a is derived based on a different estimated motor performance value 52 a. For example, FIG. 5 illustrates an example based on per unit electromagnetic torque computed using stator flux values computed from measured currents and commanded voltages using the component 53, and FIG. 6 shows an example of a per unit estimated torque value computed from measured currents and measured voltages according to computed rotor flux estimate values. As seen in FIG. 7, another possible implementation provides an estimated power factor value 52 a (PF) computed from measured currents and measured voltages according to real and imaginary power values P and Q from the component 53. FIG. 8 shows an implementation in which these real and imaginary power values are computed from measured currents and commanded voltages by the component 53, and the component 54 computes an estimated power factor angle value θ_(PF) 52 a. Yet another example is shown in FIG. 9, in which the FM signal generator 52 provides a per-unit power estimate value 52 a (P_(pu)) upon which the frequency modulation value 25 a computation is performed. Other possible examples include computation of an estimated performance value 52 a as an error between the open loop angle and an estimated rotor position.

Referring now to FIGS. 11 and 12, graphs 60 and 70 respectively illustrate motor speed 62, 72, current 64, 74 and load torque 66 and 76 curves for the motor drive 10 without and with the stability control concepts of the present disclosure for a step load change. As seen in the graph 60 of FIG. 11, absent the stability control advantages achieved by the frequency modulation adjustment concepts discussed above, and particularly for low motor speeds, the onset of a step load increase in the load torque causes the open loop (sensorless) operation to suffer from potentially significant instability or “hunting” in the motor speed curve 62, resulting in oscillation in the load torque curve 66. In FIG. 12, however, the graph 70 shows that the load torque increase in curve 76 results in a temporary drop and subsequent controlled recovery in the motor speed curve 72 (assuming a constant desired motor speed setpoint value). In this regard, the controlled modulation of the frequency setpoint via the adjusted frequency setpoint value 31 a effectively counteracts the effects of instability or hunting by compensation through the essentially out of phase frequency modulation signal 25 a. In this manner, instability is mitigated, thereby preventing or reducing the likelihood of excessive motor speed or position hunting and reducing the potential for undesired motor stoppage.

Referring also to FIG. 13, a method 100 is presented for sensorless motor speed control in a motor drive (e.g., motor drive 10 above). Although the exemplary method 100 is depicted and described in the form of a series of acts or events, it will be appreciated that the various methods of the disclosure are not limited by the illustrated ordering of such acts or events except as specifically set forth herein. In this regard, except as specifically provided hereinafter, some acts or events may occur in different order and/or concurrently with other acts or events apart from those illustrated and described herein, and not all illustrated steps may be required to implement a process or method in accordance with the present disclosure. The illustrated methods may be implemented in hardware, processor-executed software, or combinations thereof, in order to provide sensorless motor control as described herein, and various embodiments or implementations include non-transitory computer readable mediums having computer-executable instructions performing the illustrated and described methods. For instance, the method 100 may be implemented using one or more processors associated with the controller 20, by executing instructions stored in an electronic memory operatively associated with the controller 20.

The method 100 begins at 102 in FIG. 13, with receipt of the frequency or speed setpoint f* (e.g., 21 above) representing the desired operating speed of the motor load 6. The setpoint 21 can be received at 102 from any suitable source, such as an input from another element in a distributed control system, user interface, etc. At 104, one or more inverter output currents are measured or sampled (e.g., feedback signals 28 representing the three-phase output currents i_(a), i_(b), i_(c) provided by the inverter 14 and sensed using the current sensors 27 above). The sampled output currents, moreover, can be directly proportional to the actual measured inverter output currents, or the signals 28 can be filtered, shifted, or otherwise undergo any suitable signal processing such that the values 28 represent the current output provided by the inverter 14 to facilitate computation of one or more estimated motor operating parameters as discussed above. At 106 in FIG. 13, a frequency modulation value is computed (e.g., Δf* signal or value 25 a above) at least partially according to the AC output current feedback and also according to one or more voltage command signals or values (e.g., v_(a)*, v_(b)*, v_(c)* above). In this regard, the voltage command signals or values can be any voltage command setpoints in any suitable reference frame (e.g., stationary reference frame, synchronous reference frame, etc.) which are used or usable for controlling operation of the inverter. In one example, for instance, the previously computed voltage command signals or values v_(a)*, v_(b)*, v_(c)* used in a previous control cycle can be used in computing the estimated motor operating parameter for use in offsetting the frequency or speed setpoint for the next or subsequent control cycle. At 108, the frequency or speed setpoint 21 (f*), whether previously rate limited or not (e.g., rate limited frequency setpoint f_(RL) 31 above), is adjusted at least partially according to the frequency modulation value Δf*, for example, by subtracting the frequency modulation value Δf* from the original or rate limited frequency or speed setpoint 21, 31, in order to provide an adjusted setpoint f_(AD). Using the adjusted frequency or speed setpoint f_(AD), the inverter 14 is controlled at 110, after which the process 100 repeats as shown.

The above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, systems, circuits, and the like), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component, such as hardware, processor-executed software, or combinations thereof, which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the illustrated implementations of the disclosure. In addition, although a particular feature of the disclosure may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Also, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”. 

The following is claimed:
 1. A method for sensorless motor speed control in a motor drive, the method comprising: receiving a frequency or speed setpoint value representing a desired motor speed for a driven motor; sampling at least one AC output current feedback signal or value from an output of an inverter of the motor drive; computing a frequency modulation value based on the at least one AC output current feedback signal or value and on at least one voltage command or feedback signal or value; adjusting the frequency or speed setpoint value based at least partially on the frequency modulation value to provide an adjusted frequency or speed setpoint value; and controlling the inverter according to the adjusted frequency or speed setpoint value.
 2. The method of claim 1, wherein adjusting the frequency or speed setpoint value comprises: rate limiting the frequency or speed setpoint value to provide a rate limited frequency or speed setpoint value; and subtracting the frequency modulation value from the rate limited frequency or speed setpoint value to provide the adjusted frequency or speed setpoint value.
 3. The method of claim 1, wherein adjusting the frequency or speed setpoint value comprises subtracting the frequency modulation value from the frequency or speed setpoint value to provide the adjusted frequency or speed setpoint value.
 4. The method of claim 3, wherein computing the frequency modulation value comprises: computing an estimated motor performance value based on the at least one AC output current feedback signal or value and on the at least one voltage command signal or value; computing the frequency modulation value based at least partially on the estimated motor performance value.
 5. The method of claim 4, wherein computing the frequency modulation value comprises: high pass filtering the estimated motor performance value to provide a filtered estimated motor performance value; and computing the frequency modulation value based at least partially on the filtered estimated motor performance value.
 6. The method of claim 4, wherein computing the estimated motor performance value comprises computing an estimated torque value, and wherein the frequency modulation value is computed at least partially according to the estimated torque value.
 7. The method of claim 4, wherein computing the estimated motor performance value comprises computing at least one of an estimated power factor value, an estimated power factor angle value, and an estimated power value, and wherein the frequency modulation value is computed at least partially according to the at least one estimated power factor value, power factor angle value, or power value.
 8. The method of claim 1, wherein computing the frequency modulation value comprises: computing an estimated motor performance value based on the at least one AC output current feedback signal or value and on the at least one voltage command or feedback signal or value; computing the frequency modulation value based at least partially on the estimated motor performance value.
 9. The method of claim 8, wherein computing the frequency modulation value comprises: high pass filtering the estimated motor performance value to provide a filtered estimated motor performance value; and computing the frequency modulation value based at least partially on the filtered estimated motor performance value.
 10. The method of claim 8, wherein computing the estimated motor performance value comprises computing an estimated torque value, and wherein the frequency modulation value is computed at least partially according to the estimated torque value.
 11. The method of claim 10, wherein computing the estimated torque value comprises: computing at least one estimated stator or rotor flux value based on the at least one AC output current feedback signal or value, the at least one voltage command or feedback signal or value, and on at least one resistance value; and computing the estimated torque value based on the at least one estimated flux value, the at least one AC output current feedback signal or value, and on a rated torque value.
 12. The method of claim 8, wherein computing the estimated motor performance value comprises computing at least one of an estimated power factor value, an estimated power factor angle value, and an estimated power value, and wherein the frequency modulation value is computed at least partially according to the at least one estimated power factor value, power factor angle value, or power value.
 13. A non-transitory computer readable medium with computer executable instructions for sensorless motor speed control in a motor drive, the computer readable medium comprising computer executable instructions for: receiving a frequency or speed setpoint value representing a desired motor speed for a driven motor; sampling at least one AC output current feedback signal or value from an output of an inverter of the motor drive; computing a frequency modulation value based on the at least one AC output current feedback signal or value and on at least one voltage command or feedback signal or value; adjusting the frequency or speed setpoint value based at least partially on the frequency modulation value to provide an adjusted frequency or speed setpoint value; and controlling the inverter according to the adjusted frequency or speed setpoint value.
 14. A motor drive, comprising: an inverter including a plurality of switching devices operable according to switching control signals to provide AC output power to drive a motor load; and a sensorless motor speed controller providing the switching control signals to the inverter to regulate the AC output power at least partially according to a frequency or speed setpoint value, the sensorless motor speed controller comprising: a signal generator component operative to compute a frequency modulation value based on at least one AC output current feedback signal or value representing at least one AC output current of the inverter and on at least one voltage command or feedback signal or value used in controlling the inverter, an adjustment component operative to adjust the frequency or speed setpoint value based at least partially on the frequency modulation value to provide an adjusted frequency or speed setpoint value, and a command generator component operative to provide the switching control signals to the inverter at least partially according to the adjusted frequency or speed setpoint value.
 15. The motor drive of claim 14, wherein the signal generator component is operative to compute an estimated motor performance value based on the at least one AC output current feedback signal or value and on the at least one voltage command or feedback signal or value, and to compute the frequency modulation value based at least partially on the estimated motor performance value.
 16. The motor drive of claim 15, wherein the sensorless motor speed controller comprises a high pass filter operative to high pass filter the estimated motor performance value to provide a filtered estimated motor performance value; and signal generator component is operative to compute the frequency modulation value based at least partially on the filtered estimated motor performance value.
 17. The motor drive of claim 15, wherein the signal generator component is operative to compute the estimated motor performance value as an estimated torque value, and to compute the frequency modulation value based at least partially on the estimated torque value.
 18. The motor drive of claim 15, wherein the signal generator component is operative to compute the estimated motor performance value as at least one of an estimated power factor value, an estimated power factor angle value, and an estimated power value, and to compute the frequency modulation value based at least partially on the at least one estimated power factor value, power factor angle value, or power value.
 19. The motor drive of claim 14, wherein the sensorless motor speed controller comprises a rate limiter component operative to rate limit the frequency or speed setpoint value to provide a rate limited frequency or speed setpoint value; and wherein the adjustment component is operative to subtract the frequency modulation value from the rate limited frequency or speed setpoint value to provide the adjusted frequency or speed setpoint value.
 20. The motor drive of claim 14, wherein the adjustment component is operative to subtract the frequency modulation value from the frequency or speed setpoint value to provide the adjusted frequency or speed setpoint value. 